Artificial satellites in telecommunication

A daily study spanning a month of the shallow water acoustic channel was conducted to
estimate the environmental influence on performance of an underwater acoustic communications
system. An automated acoustic modem transmitted phase-coherent modulated sequences of
identical data with 186 dB re IpPa source level, at coded rates from 4000 to 16000 bits/s with
4 or 8 kHz symbol bandwidth, three times daily for a month. A 64 channel Mills-Cross receiver
array was used with horizontal and vertical beams each containing 32 and 33 elements
respectively, spaced 0.03 meters apart, with a sampling frequency of 72 kHz. Source and
receiver were deployed at depths of 20 meters respectively, with a 720 meter separation range.
Environmental measurements of wind velocity and direction, surface wave activity, current and
sound velocity profiles, and tidal measurements were performed. Results demonstrate reliable
achievement of high data-rate shallow water acoustic communications using phase-coherent
modulation techniques.

The rapid growth of satellite services using higher frequency bands such as the Ka-band has highlighted need for analyzing effects of different propagation phenomena. Since the wavelength of radiowaves is comparable with the size of rain drops, rain attenuation is the dominant propagation impairment at Ka frequencies. In addition, other impairments such as gaseous absorption, cloud and fog attenuation, tropospheric refractive effects, as well as depolarization become increasingly important with increasing operating frequency. Theoretical background of radiowave propagation principles, rain systems and gases in the atmosphere are presented to insure comprehension of propagation effects on space communication in Ka-band. Models for predicting rain attenuation and other propagation impairments along Earth-satellite path are provided in order to simplify design of communication systems. Propagation phenomena are explained on example of three propagation experiments performed in U.S., Europe and Japan. Whenever possible, mitigation techniques to overcome severe attenuations are introduced.

To observe the effects of satellite transmission on video compression technology designed at FAU's Imaging Systems Lab; an interface was designed to accept data directly from a video encoder or a 16 GByte RAID storage device. The design uses a Xilinx XC4005E field programmable gate array. The interface connects to a high speed enhanced parallel port at the computer backplane. Data stored via the interface on the computer; is transferred at a T1 rate through the ACTS T1-VSAT satellite link. In loop-back mode the data is stored, then evaluated.

The Advanced Communications Technology Satellite propagation experiment was designed by NASA to study the effects of precipitation, primarily rain, on Ka frequency band signals. Two beacon signals, transmitted from the satellite, provide attenuation data that is recorded by a propagation terminal located in Tampa, Florida. The received beacon data contains a DC bias and diurnal effects and is therefore uncalibrated. Radiometers, centered at each beacon carrier frequency, are used to set the 0 dB reference level for the beacon data, using constants determined through radiometer calibration techniques. The details of this process are examined using actual propagation data.

A test system was designed to determine the performance a QPSK satellite burst modem when using the Ka band link of the Advanced Communications Technology Satellite. Interface circuitry, largely based on the 22V10 programmable logic device, was designed to allow the modem to be controlled by a personal computer. Communication between the interface and the computer was accomplished through the computer's parallel port. TDMA frame timing was automatically controlled by the interface. A C language program provided operator control of the interface itself. Tests using this system showed that a severe night-time fading problem is experienced at the FAU receiver site. Very low error rates were recorded by this system in a loop-back transmission at the NASA satellite control terminal.

This dissertation is concerned with studies on the performance aspects of mobile LEO satellite cellular systems. Relevant performance measures studied in the dissertation include new call blocking probability, handoff failure probability, call termination probability, and call dropping probability. The analytical teletraffic models available in the literature are generalized in this work in order to characterize the 3G systems more realistically. The effect of earth rotation on cell residence time is also investigated and a new cell residence time model is developed. The new model proposed uses right-truncated gamma distribution to describe the statistics of residence time in the origination cell; and, the generalized beta distribution is used to model the residence time in subsequent cells. A closed-form expression to determine the premature call termination and call dropping probabilities is proposed. The proposed expression requires only the cumulative distribution function of the call holding time and the first four moments of the cell residence time. In all the above probabilistic considerations, the arrival process is regarded as: (i) Poissonian implying voice-like traffic and (ii) non-Poissonian depicting inhomogeneous mix of voice, data, and video transmissions expected on a trunk traffic. The effect of factors that may affect the signal intensity on the performance of mobile LEO satellite cellular systems is also investigated. In particular, a mathematically tractable expression to approximate the moments of cell residence time in origination cell as well as in subsequent cells is developed based on the received signal power.

A general and broad new class, ART, consisting of adaptive hybrid multiple access protocols applicable to communication networks sharing a single transmission medium and which behave like A loha-type, R eservation and T DMA basic protocols at low, medium and high throughput respectively, is proposed. Each hybrid protocol in this class is a combination of one protocol, possibly individually adaptive or improved version, from each of these three basic classes of protocols. Specifically, for satellite communication networks, a sub-class of ART protocols called Adaptive Satellite Hybrid Access (ASHA)+ is proposed.Two ASHA protocols named ASHA1 and ASHA2 that combine the features of S-ALOHA, TDMA-Reservation and TDMA protocols by using a proposed generalized design technique, are described in detail and their analytical models are formulated. The ASHA protocols are analyzed using Equilibrium Point Analysis technique and then their analytical results are presented. A fairly sophisticated and new simulator, modeled basically as a 3-state finite state machine, to simulate the ASHA protocols is designed, described and implemented. A new phenomenon called "protocol oscillations" is observed, in which a hybrid protocol oscillates back and forth between its two adjacent states without accomplishing much, and becomes literally unstable. Traffic measures and threshold values to remedy this problem are discussed. As a by-product of simulator validation, it is found that SRUC protocol, previously thought to be stable at all traffic levels, also suffers from this problem. In addition, some previous simulation results of SRUC protocol are felt not to be correct, and its simulation results that we believe to be correct instead, are given. Detailed simulation results of ASHA protocols are presented and also compared with their analytical results. These results provide a large amount of practical and valuable insight, heretofore unknown, into the workings of adaptive hybrid protocols in general and of ASHA protocols in particular. Moreover, these results show that ASHA protocols provide better delay versus throughput performance over the entire range of throughput compared to any of their individual constituent protocols, and extend the channel capacity to unity. ftn$\sp+$As an aside, ASHA means "hope" in Hindi language.